Method of producing metal powders



June 5, 1956 MJEISENBERG 2,749,234

METHOD OF PRODUCING METAL POWDERS Filed Aug. 13, 1953 FERROUS 5U LFAT E SOLUTION DEAERATOR somum ANALGAII 4 i SODlUM SULFATE MERCURY TANK nzcovsav rowan I ,TO mzacum' RESERVOIR more AMALGAM SUSPENSION ARGON 8 usacunv OENTRIFUGE VACUUM RETORT TO msncunv nzsenvom msncunv ARGON VACUUM RETORT BALL mu.

PURE nou POWDER INVENTOR.

PATENT AGENT METHOD OF PRODUCING METAL POWDERS Morris Eisenberg, Richmond, Calif. Application August 13, 1953, Serial No. 374,084

4 Claims. (Cl. 75-5) The present invention relates to the manufacture of metal powders. More particularly the present invention relates to the manufacture of powders of the ferrous metals.

It is an object of the present invention to provide a simple and effective method for producing fine powders of the ferrous metals.

Another object of the invention is to provide a method of producing fine ferrous metal powders of a very high degree of purity.

Still another object of the invention is to provide a simple and economically feasible method of producing ferrous metal powders which readily permits control of the particle size of the powders obtained.

More particularly it is an object of the present invention to provide a simple and economically feasible method of producing iron powders of a purity exceeding 99.9% and of a particle size within a range of from 1 to 30 microns.

In accordance with the invention 1 contact an alkali metal amalgam with a pure aqueous solution of a salt of the ferrous metal to be presented in powder form to obtain a solution and/ or colloidal suspension of the metal in mercury. I then concentrate the resultant amalgam suspension and/or solution mechanically by centrifuging and/ or vacuum filtering in an inert atmosphere that is free from oxygen to prevent the formation of any metal oxides, and thereafter distill the mercury component from the resultant concentrate while maintaining the temperature below the sintering point of the metal under treatment and while maintaining the concentrate in a state of agitation to prevent caking and lumping thereof.

To distill the mercury from the amalgam solution and/ or suspension while maintaining the temperature below the sintering point of the metal powder to be produced, I perform the distillation process under a suitable vacuum, and to provide the necessary agitation for the prevention of caking and lumping and to reduce any cakes that may form, the concentrate may be stirred during distillation, or the retort wherein the distillation is carried out may be subjected to vibrations, or all or part of the distillation process may be carried out in a rotary kiln equipped with steel balls. By adjusting the distillation temperature, the duration of the distillation process, and the amount of agitation employed in the process, I am able to predetermine the particle size of the resultant metal powders.

The flow chart shown in the accompanying drawing illustrates an exemplary process of the present invention as employed to produce a finely divided iron powder of a purity exceeding 99.9%.

An aqueous solution of ferrous sulfate, which has been purified in a conventional manner to remove all metal impurities, is passed into a deaerator 1 where most of the air dissolved therein is removed. From there the solution passes into a chamber 2 which is alternately evacuated and filled with an inert gas such as argon or nitrogen, to remove all traces of oxygen from the solunited States Patent tion. The ferrous sulfate solution, freed of oxygen in the described manner, is then introduced into, and pumped upwardly from the lower end of a reactor 3 filled with an inert gas, such as argon, against a descending stream of sodium amalgam containing from about 0.3% to 0.55% of sodium by weight which is introduced at the upper end of the reactor 3 as indicated by the arrow 4. in reactor 3 the iron of the sulfate solution displaces the sodium in the amalgam and the resultant iron amalgam suspension which is free of all oxides, due to the precautions previously taken, leaves the reactor 3 through a conduit 5 at the bottom end thereof while the displaced sodium forms an aqueous solution of sodium sulfate that leaves the reactor 3 through a discharge conduit 6 pro vided at the top end thereof. Since the aqueous solution of sodium sulfate may contain as much as 20 milligrams of mercury per litre in colloidal suspension, it may be passed through a mercury recovery tower 7. Such a recovery tower may have the form of a cylindrical column filled with copper turnings.

For best results the reactor 3 should be arranged to operate at a temperature Within the range of from 20 C. to 70 C. and the flow rates of the iron sulfate solution and the sodium amalgam introduced into the reactor should be arranged to provide a residence time within the reactor of from 2 to 10 minutes, and said rates should be related to each other in such a manner as to provide, at all times, a stoichiometric excess 0 firon ions.

As previously pointed out the iron amalgam suspension formed in reactor 3 leaves said reactor through a discharge conduit 5 provided at the bottom end thereof. The internal surface of said discharge conduit should be amalgamated to assure that the amalgam suspension passing through said conduit may thoroughly wet its inner surface and in this manner prevent effectively seepage of the sulphate solution to succeeding processing stations between the walls of the conduit and the stream of iron amalgam suspension.

In conduit 5 the iron amalgam suspension is pumped into a centrifuge 8 which operates under an argon atmosphere and wherein the iron suspended in the amalgam is concentrated in the center region due to its lighter weight. Alternatively the iron amalgam suspension leaving reactor 3 may be concentrated by passing it through a suitable vacuum or pressure filter that operates under an inert atmosphere, such as argon or nitrogen. From the centrifuge 8 or the filter (not shown) a mechanical conveyor (not shown) may be arranged to convey an amalgam containing from 3% to 10% by weight of iron through a closed conduit 9 filled with an inert gas such as argon into a vacuum retort 10, while the mercury in the peripheral region of the centrifuge 8 is returned through a conduit 11 to a mercury reservoir (not shown) to be re-used in the preparation of sodium amalgam. In the retort 10 the mercury is distilled off at a temperature below the level where sintering of iron commences, and the retort is preferably maintained in a continuous state of agitation to prevent caking and lumping of the finely divided iron particles that remain in the retort while the mercury is removed by distillation. In the exemplary process of the invention which I am about to describe, and wherein the principles of the invention are employed to produce iron powder, the mercury is distilled off at a temperature not exceeding 300 C. and ranging preferably between 200 C. and 280 C., and the retort 10 may be evacuated to a pressure equalling preferably from 10 to 30 microns of mercury.

From the retort 10 the iron powder mass remaining therein after the mercury has been boiled off may be conveyed under an inert atmosphere to a mill 12 filled with an inert gas such as argon, wherein it is subjected to the action of steel balls to break down any clusters of iron particles that may have formed in the retort and liberate any traces of mercury that may have been occluded in such clusters. From the ball mill 12 the powder is delivered through another conduit filled with an inert gas into a second retort 13 where it is again subjected to elevated temperatures under a suitable vacuum so as to drive out any residual mercury that may have been liberated from occlusion by the action of the steel balls in the mill 12.

To prevent sintering of the powder particles, the temperature to which it is subjected in the second retort 13 should again not exceed 300 C. and the pressure in the retort should preferably be of the order of from 2 to 10 microns. The powder emerging from the second retort is of very high purity exceeding 99.9% in the case of iron and of very fine particle size ranging from 1 to 30 microns. The fineness of the powder resulting from the described process may be predetermined within practical limits by proper choice of the temperature to which the mass is exposed in the retorts 1i and 13, the residence time in these retorts, and by the degree of agitation to which it is subjected in said retorts and in the ball mill 12. In general, the final product will be of a finer particle size, the lower the temperature to which it was subjected in the rctorts, the shorter the residence time, and the more vigorous the agitation to which it was subjected in said retorts and in the ball mill.

My invention is particularly advantageous for produc ing fine powders of the ferrous metals such as iron, nickel,

cobalt, and chromium which tend to sinter and would n therefore form metal sponge, instead of fine metal powders, under conventional distillation methods. However, the principles of the invention may also be usefully employed for the production of fine powders of high purity of all metals that are capable of replacing alkali metals in amalgam, such as zinc, aluminum, tin, lead, manganese, copper, silver, gold, etc; in other words the process of the invention is applicable to all metals whose polarographic half-wave potentials are less negative than -2.l5 volts, i. e., which are electrochemically more noble than sodium metal.

While I have described my invention with the aid of an exemplary embodiment thereof, it will be understood that I do not wish to be limited to the specific tempera tures, vacuum pressures and sequence of operational steps given by way of example, which may be departed from without departing from the scope and spirit of my inventlon.

What I claim is:

1. The method of producing a ferrous metal powder which comprises subjecting a purified solution of a salt of the metal, to be presented in powder form, in a confined space alternately to a vacuum and inert gas to remove oxygen dissolved in the solution, contacting the oxygen-free solution countercurrently with an oxygen-free alkali metal amalgam in an inert atmosphere, concentrat ing the resultant metal amalgam mechanically in an inert atmosphere, and distilling the resultant concentrate under agitation at a temperature below the sintering point of the metal to remove the mercury.

2. The method of producing iron powder which comprises contacting a purified oxygen-free aqueous solution of iron sulfate with sodium amalgam under an inert atmosphere, concentrating the resultant colloidal suspension of iron in mercury under an inert atmosphere, distilling the resultant concentrate under agitation in a vacuum at a temperature below 300 C., subjecting the resultant residue under an inert atmosphere to a mechanical comminution process to break apart any particles that may adhere to each other and thus free any residual mercury that may have been occluded between such particles, and again subjecting the comminuted iron particles to a distillation process in a vacuum at a temperature below 300 C. while maintaining them in a state of agitation to drive off any residual mercury.

3. The method of producing iron powder which comprises subjecting a purified aqueous solution of iron sulfate in a confined space alternately to a vacuum and inert gas to remove substantially all oxygen therefrom, contacting it countercurrently with sodium amalgam under an inert atmosphere, mechanically concentrating the resultant colloidal suspension of iron in mercury under an inert atmosphere, distilling the resultant concentrate in a vacuum under agitation at a temperature below 300 C., subjecting the resultant residue under an inert atmosphere to a mechanical comminution process to break apart any particles that may adhere to each other and thus free any residual mercury that may have been occluded between such particles, and again subjecting the comminuted iron particles to a distillation process under agitation in a vacuum at a temperature below 300 C. to drive off any residual mercury.

4. The method of producing an iron amalgam which comprises subjecting a purified aqueous solution of iron sulfate in a confined space alternately to a vacuum and an inert gas to remove substantially all oxygen therefrom and then contacting it countercurrently with sodium amalgam under an inert atmosphere.

References Cited in the file of this patent UNITED STATES PATENTS 1,736,072 Duschak Nov. 19, 1929 2,082,362 Stevens June 1, 1937 2,239,144 Dean et al. Apr. 22, 1941 FOREIGN PATENTS 296,814 Great Britain Sept. 10, 1928 

1. THE METHOD OF PRODUCING A FERROUS METAL POWDER WHICH COMPRISES SUBJECTING A PURIFIED SOLUTION OF A SALT OF THE METAL, TO BE PRESENTED IN POWDER FORM, IN A CONFINED SPACED ALTERNATELY TO A VACUUM AND INERT GAS TO REMOVE OXYGEN DISSOLVED IN THE SOLUTION, CONTACTING THE OXYGEN-FREE SOLUTION COUNTERCURRENTLY WITH AN OXYGEN-FREE ALKALI METAL AMALGAM IN AN INERT ATMOSPHERE, CONCENTRATING THE RESULTANT METAL AMALGAM MECHANICALLY IN AN INERT ATMOSPHERE, AND DISTILLING THE RESULTANT CONCENTRATE UNDER AGITATION AT A TEMPERATURE BELOW THE SINTERING POINT OF THE METAL TO REMOVE THE MERCURY. 